Electric insulating material



Dec. 14, 1948. A. A. NEW ET Re. 23,062

ELECTRIC INSULATING MATERIAL Original Fi led Sept. L6, 1942 /o 40 I0 a0 Z POL rs rm' /vg INVENTORS AA. NEW

.5. a. FOO/PD A TTORNE'Y Reissued Dec. 14, 1948 23,062- ELECTRIC INSULATING MATERIAL Archibald Alan New, Stanley =(ireorge Foord, and

Dudley Robert Beckwith, London; England,,assigners to International Standard Electric Carnotation, New York, N. Y.

Original No. 2,365,646, dated December '19, 1944,

Serial No. 458,598, September :16, 1942;. Application for reissue March 19, 1,947., No. 735,574. Great Britain 31,1941

1 Claim. (01. iii-122 i This invention relates to electr c insulating materials and more particularly tov tapes impregnoted with solid or viscousi'liquid insulating ma- The we'll known sticky tapes used for electrical insulation purposes, made with a cotton fabric and compounds comprising rubber, rubber resin, bltumens. waxes, resin, vegetable oils and the like. do not have particularly good electrical properties. sp cially under very humid conditions and/or. at high frequencies such as are used .for radio communication. Their widespread use is .based mainly on their convenience combined with moderate electrical properties, and they'sllfler from what in some circumstances, is a defect in that when exposed, o the air th y oxidise and lose their tackiness without, how ever. having more than a slight gain in hardness of the covering formed.

For some purposes a tape which is sticky when applied but will harden subsequent to application is desirable. It the object of the present invention to: provide such a tape which will in addition have excellent insulating properties even at high frequencies.

It has been proposed to build up a body of insulation around a cable joint or the like by la p g paper tapes impregnated with polystyrene and basting the tapes at intervals during the lapping operation with monomeric styrene. Ithas also been proposed to cover a cable joint with acetylated cotton tape impregnated with a rubber-polystyrene, mixture and to causethis latter' mixture to swell by coating it with a lacquer consisting of polystyrene dissolved in manuallyrene, which may contain a suitable pl'asticiser.

According to the present invention we provide a fibrous tapeimpregnated with a mixture containing a polymerized aromatic vinyl hydrocarand a monomeric aromatic vinyl hydrocarbon, in such proportions that the mixture is suificiently viscous to render the tape. sticky, when first used, the whole gradually hard'ehs with time.

The mixture with which the tape is impregnoted may contain rubber, a polymerized aromati'c vinyl hydrocarbon, such as polystyrene, and between and 50% of a monomeric aroma-tic vinyl hydrocarbon capable of polymerisation such as n'ionostyrene.v

Such a tapeis sticky because of the action of the monomer in swelling the rubber andv the polymer and can be lapped over an article to beinsnlated; e. a power cable joint withoutthe use or adhesiv Percentages of the monomer below the, lower limit stated above are insufiicient to swell the rubber and polymer satisfactorily, whilst if percentages above the higher limit stated are used the mixture is insufllcicntly viscous and may flow away from the tape. I

As an example, tape composed of acetylatcd cotton, preferably having "between 30% and combined acetic acid content, is impregnated with the mixture of rubber, polystyrene, polyisobutylene and polyethylene described in British Patent53'4A64 and, prepared as therein described. This mixture contains. 51.6% polyst'y': rene, 23.2% crpe rubber, 21.4% po1yisobutylene and 3.8% polyethylene. The tape is impregnated by any well-known, method such as spreading, or rolling or frictioning' on hot calender rolls. The tape so impregnated is not sticky. It is kept in rolls until shortly before it is. required for use. It -is then passed through a. bath con taining monostyrene during, a time or immersioii of 15 seconds during which it takes up mono? styrene to the extent of 2.0% of the I'n'ixtureor'iginal-ly in the tape... The tapepis rewound after passage through the bath and during a period of several weeks remains sticky (if kept in airtight containers) and capable of being unwound to be rewound round an article to be insulated.

Other examples according to the invention comprise fibrous tapes impregnated with mixtures of monostyrene and polystyrene, or with mixtures of monostyrene, polystyrene and a nonpolymer'isable plastlciser' for polystyrene. In order that a tape should be sticky, the viscosity of the mixture with which it, is impregnated should not be less than 10 poises and as the tape in accordance with the invention must be sticky at room temperature, the impregnating mixture should have a viscosity of not less than 10 poises at room temperature, i. e. 20 C. Also the. viscosity of the impregnating mixture must not exceed 10 poises at, 20 C. This figure is approximate since; it is very difficult to measure viscosities in the neighbourhood of this figure, but a viscosity oflO noises approximately is the upper limit for a tape impregnated therewith to be sticky. Another criterion is that the tape must harden with time, by polymerisation of the monostyrene, i. e, the viscosity of the mixture must become greater than 10. p'oises at, room temperature and preferably much greater.

The precise composition of a mixture of monostyrene. polystyrene and non-polymerisa'ble. plastici's'er for polystyrene as imiire nant f a fi r t p according to the invention depends. upo

the triangular diagram commonly used when" dealing with mixtures of three components. The left point of the base represents monostyrene, the right point polystyrene, and intermediate points mixtures of monostyrene and polystyrene.

A mixture represented by point A is 50% monostyrene 50% polystyrene and has a viscosity of 10 poises. A mixture represented by point C is 20% monostyrene and 80% polystyrene and has a viscosity of 10 poises approximately. Consequently any mixtures represented by points on the line AC inclusive of A and C are suitable as impregnants of tape according to the invention. These mixtures gradually harden with time so that the mixtures'at times after being formed are represented by points on the base to the right of the original point, the point representing the mixture moving with time until it coincides with the right end of the base 100% polystyrene.

The vertex of the triangle represents 100% non-polymerisable plasticiser and points in the triangle outside the base representmixtures of monostyrene, polystyrene and non-polymerisable plasticiser. The viscosity of the mixture represented by any such point depends, as stated above, upon the viscosity of the plasticiser and upon its plasticising effect.

For the case of. a polymerised alpha methyl styrene having a viscosity of 30 centipoises at 20 (3., point B (i. e. 90% plasticiser 10% polystyrene) represents a mixture which has a viscosity at 10 poises at 20 C. and'point D (50% plasticiser 50% polystyrene) a mixture which has a viscosity of 10 poises approximately at 20 C. Any point on line AB represents, for this particular plasticiser, a mixture having a viscosity of 10 poises at 20 C. and any point on line CD, a mixture having a viscosity of 10 poises, approximately at 20 C. A mixture represented by any point such as E, within the triangle will gradually alter in composition (and viscosity) with time in such a way that the mixtures at times after being first formed are represented by points situated successively to the right on the horizontal line through the point representing the first formed composition. A mixture represented by point E eventually becomes a mixture represented by point D, by polymerisation of the monostyrene, Thus the line ED is one limit of the area within which mixtures that are impregnants of tape according to the invention must lie, and this area is that of the figure AEDC (when the above mentioned polymerised alpha methyl'styrene is used as non-polymerisable plasticiser). Preferably, in order that the mixture should with time attain a viscosity much greater than 10 poises, the limiting line of the area should be GH, parallel to ED through a point H on CD representing a composition containing approximately 5% monostyrene.

The effect of the use of a plasticiser for polystyrene of a different viscosity but similar plasticising effect to the plasticiser above mentioned upon the composition of mixtures to be used as impregnants of tapes according to the invention may be seen from another example. The points B and D represent mixtures of polystyrene and a polymerized alpha methyl styrene viscosi y Qi 4 100 poises at 20 C. that have viscosities of and approximately 10 poises at C. respectively. Thus if polymerised alpha methyl styrene of a viscosity of 100 poises is used, mixtures to be used as impregnants of tapes according to the invention are represented by points in the area AE'D'C. Preferably also the boundary line is drawn parallel to ED and nearer to the base of the triangle (so as to include only mixtures that harden to a viscosity much greater than 10 poises) in the same manner as previously ex- 1 plained.

The slopes of the lines AB. AB, CD and CD take into account the fact that monostyrene is a much better plasticiser for polystyrene than any other plasticiser, so that for example a mixture of 50% monostyrene, 50% polystyrene has a viscosity of 10 poises, but to attain an equally low viscosity with a mixture of polymerised alpha methyl styrene and polystyrene, at least 90% of the former is necessary.

Other plasticisers that may be used in carrying out the present invention are purified ethyl oleate or any of the plasticisers for polystyrene i used according to British Patent 490,814. As an example of one of the latter plasticisers, alpha isoamylnaphthalene may be taken, and as this has a viscosity of about centipoises at 0 C.

, this example will illustrate the efiect upon the composition of mixtures to be used in impregnating tape according tothe invention of the dif ferent plasticising effect of different plasticisers. Referring again to the drawing the point B" represents a mixture of alpha isoamylnaphthalene and'polystyrene' 25% that has a viscosity of 10 poises at 20 C. and point D" represents a mixture of the same two substances that has a viscosity of approximately 10 poises. This last mentioned mixture contains 45% polystyrene. Thus mixtures of monostyrene, polystyrene and alpha isoamylnaphthalene that may be used for impregnating tapes according to the present invention fall within the area AE"D"C on the diagram. Preferably the boundary line of this area is drawn below E"D in the manner already explained.

The above examples are correct for polystyrene of molecular weight of 80,000 to 100,000 but polystyrenes of other molecular weights may be used with corresponding changes in the limiting compositions. Moreover polystyrene in any of the above examples may be replaced by a mixture of polystyrene and rubber or by any other polymerised aromatic vinyl hydrocarbon or by a mixture of a polymerised aromatic vinyl hydrocarbon and another polymerised substance, with gorresponding changes in the limiting composilOIlS.

The tape according to examples described with reference to the drawing may be prepared by any of the following methods, the precise method to be adopted and the time of impregnation according to any of the methods being dependent upon the viscosity of the impregnant at the temperature employed during the impregnation. The methods which may be used are (a) direct impregnation of the tape in roll form with the mixture at elevated temperatures and either with or without pressure difference to force the mixture into the r011, (22) direct impregnation of a roll with monostyrene or a mixture of monostyrene plus one of the'plasticisers mentioned at ordinary temperatures followed by heating for sufficient time to cause the required proportion o monostyrene to polymerise, (c) rolling or fric- 5 tloning the mixture into the tape or (d) passing a, tape containing polystyrene or a mixture or polystyrene and non-polymerisable plasticiser through a bath of monomeric styrene or a bath containing a mixture of monomeric styrene and a non-polymerisable plasticiser.

Tape so prepared may be wound into rolls and if kept in air tight containers will retain its stickiness for some months sufiiciently to enable it to be unwound and rewound round an article to be insulated.

Tapes made in accordance with the present invention have all the convenience of application of well known sticky tapes and have better electrical properties particularly at high humidities and at radio frequencies. The lappings of tape adhere together to produce a unitary structure which is immediately waterproof and thereafter gradually harden to a solid mass by polymerisation of the monomer used and aided to some extent by evaporation also. Instead of the usual black shade of present sticky tapes, a

wider range of colouring is possible with tapes according to the invention. Indeed if rubber is not used, tapes according to the invention can be made of any desired colour, by the use of coloured tapes as the fibrous base and/or by the addition of small amounts of suitable dyes or pigments to the impregnating materials.

What is claimed is:

Fibrous tape impregnated with a mixture of 51.6% polystyrene, 23.2% rubber, 21.4% polyisobutylene, 3.8% polyethylene and monostyrene to the extent of 20% of said mixture, said mixture having a viscosity of between about 10 and about 10 poises at about 20 C. and the viscosity of said mixture becoming with time greater than about 10 poises at about 20 C.

ARCHIBALD ALAN NEW. STANLEY GEORGE FOORD. DUDLEY ROBERT BECKWITIH.

No references cited. 

